Interpretive Summary: Bacteria transport experiments were performed using growing and resting cells at two different water saturations (80 and 40%). Results from this study clearly indicated that the growth stage of the cells influenced their transport through sand, because differences in cell surface chemistry. The growing cells exhibited less transport potential, especially at lower water saturations, and continuously released low concentrations of cells into flowing water. The collected transport data were described using a model that simultaneously considers both chemical interactions between bacteria and sand, and retention of bacteria in small soil pores. New concepts and hypotheses were formulated in this model to include biological aspects associated with bacteria growth in soils.

Technical Abstract:
Bioremediation is a cost efficient clean-up technique that involves the use of metabolically active bacteria to degrade recalcitrant pollutants. To further develop this technique it is important to understand the migration and deposition behaviour of metabolically active bacteria in unsaturated soils. Unsaturated transport experiments were therefore performed using Deinococcus radiodurans cells that were harvested during the log phase and continuously supplied with nutrients during the experiments. Additional experiments were conducted using this bacterium in the stationary phase. Two different water saturations were considered in these studies, namely 80 and 40%. Results from this study clearly indicated that the physiological state of the bacteria influenced its transport and deposition in sands. Metabolically active bacteria were more hydrophobic and exhibited greater deposition (attachment and straining) than bacteria in the stationary phase, especially at a water saturation of 40%. The breakthrough curves for active bacteria also had low concentration tailing as a result of cell growth and release from the solid phase. Collected breakthrough curves and deposition profiles were described using a model that simultaneously considers both chemical attachment and physical straining. New concepts and hypotheses were formulated in this model to include biological aspects associated with bacteria growth inside the porous media.